1. Field of the Invention
This invention relates to rotary printing presses. More particularly, the invention relates to intermittent motion rotary printing presses for printing on one side successive definite lengths of a continuous flexible web. The invention has particular utility with flexographic presses, but also relates to other rotary presses, such as offset presses.
2. Description of the Prior Art
Rotary presses are commonly used to print continuous flexible webs. Such presses generally include an inking cylinder, a print cylinder and a pressure cylinder, all mounted for rotation about parallel horizontal axes. In operation the rotating inking cylinder surface picks up printing ink from a suitable supply source and transfers it by surface contact to a printing plate mounted on and forming a part of the surface of the print cylinder, which is rotating at the same surface speed but in opposite direction to the inking roll. The printing plate, which contains the raised letters or design to be applied to the web, can be formed from rigid curved metal plates which conform to the print cylinder, or it can be a flexible mat of rubber or the like which is adhered to the print cylinder magnetically or with appropriate adhesives.
The web is printed by advancing it past the inked surface of the rotating printing plate at a speed equal to the surface speed of rotation of the plate, while using the pressure cylinder to confine the web in printing contact with the printing plate.
In continuous printing operations, such as where a pattern is printed uninterruptedly on a web, the three cylinders rotate concurrently, the inking cylinder remaining in inking contact with the printing plate and the pressure cylinder confining the web in printing contact with the plate, while the web is advanced through the press without pause. However, many printing operations involve printing of successive definite lengths of web in separate steps, with web travel stopped between each printing step; for these operations, intermittent motion presses are used.
In most intermittent motion presses, the step of inking the printing plate by the inking cylinder is done prior to and independently of the step of printing the web; i.e. the inking step is done while web travel is stopped between printing steps. Such presses are constructed and arranged so that during inking the pressure cylinder and web are moved away from the printing plate, allowing the print cylinder to rotate for inking; for the ensuing printing step, the inking cylinder is moved away from the printing plate and the pressure cylinder moved into position to confine the PG,4 web in printing contact with the plate. Generally, the inking cylinder continues to rotate in contact with the ink supply whether or not it is in contact with the printing plate, in order to keep fresh ink on its surface.
Web advancement in intermittent motion presses can be accomplished in several ways. In an internally powered arrangement, which is the most common, the print cylinder and pressure cylinder are powered for rotation by means within the press, one cylinder often being driven by the other which is in turn rotated by the power means, and the web is frictionally "pinched" between either the print and pressure cylinders or a pair of powered "nip and pinch" rolls and advanced when they rotate during the print step. In an externally powered arrangement, the web is pulled through the press by advancing means separate from and downstream of the press, and its travel path causes it to frictionally contact enough of the pressure cylinder surface so that during the print step the web rotates the pressure cylinder, and generally also the print cylinder which is suitably linked to the pressure cylinder. A third arrangement combines the first two--i.e. external power means advance the web and help to rotate the pressure and print cylinders while means within the press impart supplementary rotational power to one or both of the print and pressure cylinders.
Intermittent motion presses of the type described must include means for separating the pressure cylinder and printing plate during each inking step and for bringing them back into proper web-confining relationship during each printing step. Such presses likewise must include means for maintaining proper inking contact between the inking cylinder and printing plate during each inking step and for separating them during each printing step. Heretofore several approaches to these requirements have been used.
One approach is to maintain the pressure and inking cylinders stationary and mount the print cylinder on shiftable means whereby the printing plate can be moved between positions in or out of contact with one or both of the inking and pressure cylinders; Walther U.S. Pat. No. 5,031,532 discloses such an arrangement, although for a continuous press. This approach, however, often requires complex mechanisms and controls to provide precision in positioning the print cylinder in relation to the pressure cylinder for proper print quality. Additionally, it does not provide as much rigidity of print cylinder mounting as is the case when the print cylinder can be mounted in fixed supports.
A second approach to cylinder positioning in intermittent motion presses has been to mount one or both of the inking and pressure cylinders on separate shiftable supports which are independently moved between inking step and printing step positions by appropriate means; McAnelly U.S. Pat. No. 4,747,347 discloses a press in which the pressure cylinder is moved in this manner, although no provision is made in that patent for motion of the inking cylinder. Here the problem is also one of complexity of construction, since two separate systems must be used for both mounting and moving the cylinders and for accurately controlling the pressure each exerts on the print cylinder or web, as applicable, when in contact therewith.
Marozzi et al. U.S. Pat. No. 5,224,422 discloses yet another approach to the problem of cylinder positioning. In this approach the inking cylinder and pressure cylinder are mounted on diametrically opposite sides of the print cylinder and the printing plate extends less than half way around the print cylinder circumference, whereby whenever the plate is in contact with one of the pressure and inking cylinders it is out of contact with the other. With this approach all three cylinders can remain in fixed positions since separation is accomplished by the geometry of the system. However, the Marozzi, et al. approach limits the length of pattern which can be printed on the web to at most half the circumference of the print cylinder; if longer patterns are required, the printing plate length must be extended circumferentially around the print cylinder and the mechanism becomes one where at least one of the print or inking cylinders must be shifted between two positions during operation of the unit.